1. Field of the Invention
This invention relates to an economical process for producing medium carbon, high chromium molten iron which is obtained as an intermediate in the production of high chrome steels such as stainless steel and heat resisting steel.
2. Description of the Prior Art
Following the recent development and commercial operation of second refining processes such as the vacuum oxygen decarbonization (VOD) method and argon oxygen decarbonization (AOD) method that rely on preferential decarbonization, steel makers are taking more interest in the production of high chromium crude melt to lower the cost of producing stainless steel.
Japanese Patent Publication No. 4486/77, entitled "Process for producing stainless steel", invented by Toyosuke Tanoue et al. and filed by Sumitomo Metal Industry Co., Ltd. describes a process for producing stainless steel using high carbon ferrochrome as a chromium source. This prior invention uses a chromium source composed of high carbon chromium in a hot molten state and it essentially differs from the process of the present invention which starts with using solid high carbon ferrochrome at ordinary temperature as a chromium source. The reasons are:
(1) It is not economically feasible to obtain hot materials at a stainless steel mill, so it is generally difficult for the process of the prior invention to obtain high carbon ferrochrome in a molten state;
(2) Therefore, to perform that process, it is generally necessary to use solid high carbon ferrochrome at ordinary temperature, which is then heated in an electric furnace to a temperature higher than its melting point to form high carbon ferrochrome in a molten state.
The present invention manipulates the carbon and chromium contents (in wt%) in the molten iron so as to dissolve solid high carbon ferrochrome in the melt at a temperature lower than its melting point (i.e. without being heated to that point). Hence, the concept of the present invention differs entirely from that of the prior invention. The course followed by the Sumitomo process is Route I as depicted in FIG. 10 of the accompanying drawings. Route II in that figure represents the melting of solid high carbon ferrochrome to provide molten, high carbon ferrochrome. The route taken by the present process is not II.fwdarw.I but III which indicates that solid high carbon ferrochrome is taken into solution by the melt at a temperature lower than the melting point of the ferrochrome.
Japanese Patent Application (OPI) No. 108116/75 (the term "OPI" as used herein means a Japanese unexamined published patent application), entitled "Process for producing low carbon chromium steel and ferrochrome alloy", aims at producing high-grade chromium steel of low carbon content (less than 0.05%) by refining high carbon chrome-bearing molten metal with oxygen without having the difficulties encountered in the known methods, i.e. formation of slag that contains an undesired amount of chromium and attack of the refractory lining of the refining vessel. To reduce the chromium content in the slag, such invention locates one or more nozzles around each oxygen injecting nozzle through which to supply a solid carbon carrier in dust form below the surface of the molten metal bath so that the oxygen jet is shrouded by a jacket of the fine particulate carbon carrier. In this prior art process, carbon is supplied to prevent oxidation of chrome. In this process, if an oxygen jet is supplied directly to the molten metal to produce high-grade chromium steel of a carbon content lower than 0.05%, oxidation of chromium occurs, and to prevent this, the process supplies a jacket of fine particulate carbon carrier that shrouds the oxygen jet.
In the present process, carbon is not used to prevent oxidation of chromium, but rather is supplied as a heat source to dissolve a solid chromium source such as high carbon ferrochrome at low temperatures in the medium carbon melt in a smelting furnace. The melt produced by the present process contains a medium amount of carbon. When a carbon source is supplied to the furnace from below in the present process, it is supplied through a basal tuyere of triple concentric tube type composed of an inner tube for supply of carbon, an intermediate tube for supply of oxygen and an outer tube for supply of a coolant. Hence, the present process contemplates an intermediate having a different carbon level than that of the product contemplated by the prior art process, and since carbon is blown for different purposes in the two processes, the construction of the basal tuyere used also differs between the two processes. Therefore, the process of Japanese Patent Application (OPI) No. 108116/75 differs from the present process in object, construction and effect.
A third prior art reference is Japanese Patent Application (OPI) No. 10319/78, entitled "Metal refining method and apparatus", filed by British Steel Corporation. It describes an invention which relates to the manufacture of steel and in particular relates to a method and apparatus whereby solid iron-bearing materials can be converted to molten steel in a continuous, semi-continuous or batch mode. According to one aspect of such invention, there is provided, in a method of manufacturing steel in a metallurgical vessel containing molten ferrous metal, a process for raising the energy level within the melt comprising injecting solid carbonaceous material below the surface of the melt and introducing oxygen or an oxygen-containing gas into the vessel to react with the carbonaceous material to liberate heat. Such invention utilizes carbon to generate heat but it does not contemplate chromium-containing steel at all as a product. In contrast, the present invention contemplates high chromium crude molten iron as an intermediate product, and to manufacture it at low cost, the present invention uses the heat generated by using carbon according to the content (in wt%) of carbon and chromium in the molten steel. Therefore, the two inventions may be the same as each other in that both utilize carbon as a heat source, but they differ from each other in object, construction and effect.